I also was having the Service unavailable. So what i did was dial ##778# >edit mode>000000>Modem settings>RevA Enable>Menu button> Commit changes phone Restarts> Check for update might get a Service unavailable Check again right after and you should get it.. Worked for me Enjoy
Don't Forget to Change back to your normal 3g setting after you are done
Here is what I did in So. Cal and it worked for our two Thunderbolts:
1. Download the LTE OnOff application from the app store.
2. Launch LTE OnOff app and set the mode to "CDMA only". This will put the phone in 1x. Make a note of the default selection.
3. Check for updates now. You should be able to download the update.
4. After download completes, go back to the LTE OnOff app and put back to original cdma/LTE selection.
5. Once your back to 3G or 4G, check for updates and you should see the option to install the update!
Do this at your own risk, but it worked for me. Enjoy!